1. Technical Field
The invention relates to a charge transfer device that includes a charge transfer path that transfers electric charges, a charge branch path that is connected to the charge transfer path and alternately distributes the electric charges, which are transferred along the charge transfer path, into two directions, and two branch transfer paths that are connected to the charge branch path and are provided corresponding to the two directions.
2. Description of the Related Art
In order to enhance a frame rate in a CCD (Charge Coupled Device) type solid-state imaging device, the following method has been proposed. That is, the method outputs electric charges through multiple lines by branching a horizontal charge transfer path into two lines at an output end thereof and connecting output sections to the two lines, respectively. In such a solid-state imaging device, in order to prevent interference of signal packets, it is necessary to reliably distribute and transfer electric charges in the branch portion that branches the electric charges.
In order to solve this problem, Japanese Patent Nos. 2585604 and 2949861 (corresponding to U.S. Pat. No. 5,309,240) describe technologies that form the branch portion in a triangle shape and reduce the travel time of electric charges in the branch portion by a potential gradient using the short channel effect, thereby suppressing the interference during distribution and transfer of the electric charge.
FIG. 6 is a diagram illustrating the schematic configuration of a horizontal charge transfer device of the related art.
The horizontal charge transfer device shown in FIG. 6 includes a charge transfer path, a charge branch path, and branch transfer paths. The charge transfer path is defined by a charge transfer channel 50 and electrodes 1A, 1B, 2A, and 2B provided above the charge transfer channel 50. The charge branch path is defined by the charge transfer channel 50 and electrodes 3A, 3BP, and 3BR provided above the charge transfer channel 50. The branch transfer paths are defined by the charge transfer channel 50 and electrodes 4A and 4B provided above the charge transfer channel 50.
In the device shown in FIG. 6, in a state where electric charges are accumulated in the charge transfer channel 50 below the electrode 2B by setting a voltage φ2 to a high level and setting voltages φ1, φ1P, and φ1R to a low level, the electric charges below the electrode 2B are transferred to and accumulated in the charge transfer channel 50 below the electrode 3BP by setting the voltage φ2 to a low level and setting the voltages φ1 and φ1P to a high level. Then, the electric charges below the electrode 3BP are transferred to the charge transfer channel 50 below the electrode 4B by setting the voltage φ2 to a high level and setting the voltages φ1 and φ1P to a low level. In this way, the charges are transferred from the charge transfer path to the branch transfer path.
In the device shown in FIG. 6, electric charges which flow linearly from the right side in the figure toward the left side thereof are directed so as to flow toward the upper left side or the lower left side in the charge branch path, and then the electric charges are transferred to the branch transfer path. In FIG. 6, a circled symbol “−” denotes an electron, and an arrow denotes the maximum distance of a travel path of the electron from the charge transfer channel below the electrode 2B to the charge transfer channel below the electrode 3BP. As indicated by the arrow, it can be seen that the electron in the end of the charge transfer path travels linearly for a while due to a fringing electric field generated by the electrode 2B and the electrodes 3A and 3BP, is gradually directed to flow toward the upper left side, and travels to the charge transfer channel below the electrode 3BP. As such, in the charge transfer device that branches the charge transfer path into two lines in the charge branch path, the travel path of electric charges in the charge branch path is elongated due to its structure. As a result, the charge transfer time in the charge branch path becomes longer than the charge transfer time in the charge transfer path and that in the branch transfer paths.
Despite this phenomenon, the device shown in FIG. 6 drives the electrodes in the charge branch path at the same frequency as that for the electrodes in the charge transfer path and the branch transfer paths. Accordingly, the charge transfer time in the charge branch path is limited by the charge transfer time in the charge transfer path and that in the branch transfer paths, which makes it difficult to increase the charge transfer time. As a result charge transfer might not be completed within a predetermined transfer time, and charge transfer efficiency might be deteriorated.
If the charge transfer time in the charge branch path is made longer enough, the above concern can be removed. However, if the travel distance of electric charges in the charge branch path is excessively long, it is necessary to make the charge transfer time be longer correspondingly. Accordingly, it would be difficult to realize it.